1. Field of the Invention
The present invention relates to a method and an apparatus for oxidizing nitrides, and more particularly, to a method and an apparatus for oxidizing nitrides in order to form a structure with nano-pattern in the nitrides.
2. Description of Related Art
Owing to the excellent dielectric property, nitrides have become important materials in the manufacture process of semiconductor devices. On the other hand, nitrides often act as masks in the process of manufacturing microstructures due to its high resistance against acids and bases. In addition, nitrides have also become one of the important optical materials in recent years because of their excellent optical property. For example, silicon nitride (Si3N4) can serve as an insulating layer in semiconductor devices as well as mask layer in a CMOS manufacturing process. On the other hand, aluminum nitride (AlN) is not only used for forming insulating layers of semiconductor devices, but also for forming heat conductive layers of the same devices. Furthermore, aluminum nitride can also be utilized in the manufacturing process of UV sensors.
Although a nitride film can be widely applied, the nitride film is not easy to be patterned because of its stable chemical property. Moreover, the oxidation rate of nitrides is very slow so that complex processes are required to oxidize or modify the nitride film for subsequent application. Therefore, it is difficult to carry out the oxidation process of a nitride film. For example, the temperature for transforming silicon nitride into silicon oxide by using conventional wet thermal oxidation technique is required to be as high as 1100° C. (with reference to T. Enomoto, R. Ando, H. Morita, and H. Nakayama, Jpn. J. Appl. Phys. Vol. 17, p. 1049 (1978)). On the other hand, gallium nitride can be transformed into gallium oxide by reacting with the hot air only when the reaction temperature is higher than 900° C. (with reference to S. D. Wolter, et al., Appl. Phys. Lett. Vol. 70, p. 2156 (1997)). However, heat also decomposes nitrides and thereby the film quality deteriorates through heating (with reference to C. B. Vartuli et al., J. Vac. Sci. Technol. B 14, p. 3523 (1996)). Even though there is alternative (e.g. anode electrolysis oxidation) for transforming nitride film into oxide film without heat, however, the oxidation rate of Si3N4 in conventional anode electrolysis oxidation process is too slow (with reference to T. B. Tripp, J. Electrochem. Soc. Vol. 117, p. 157 (1970)).
U.S. Pat. No. 6,190,508 discloses a method to enhance the oxidation probability of InAlN, InGaN and GaN with exposure of light. However, since the scope of oxidation is too large, it is hard to build a micro- or nano-structure without cooperating with a photolithography process. Recently, a method to oxidize nitrides on the conductive substrate by using scanning probe lithography is also suggested, which uses a voltage of 5V at ambient temperature [F. S. Chien, et al., Appl. Phys. Lett., Vol. 76, No. 3, p. 360 (2000)]. Through the oxidation illustrated above, the line width of the resulted silicon oxide can be shorter than 100 nanometers. Subsequently, owing to the high selectivity to acids or bases, careful etching can form a structure having a high aspect ratio and a line width shorter than 100 nanometers. In other words, the desired pattern can be “written” directly without using a mask. Consequently, the manufacturing cost can be lowered significantly. Although this method can oxidizes nitrides at an ambient temperature, it is not practical for application due to the shield of electronic field and the retardation of electron flow as the thickness of the oxidized film increases. Therefore, the chemical reaction will stop and the thickness of the oxidized scope will range in a few nanometers only.